A variety of electronic devices are comprised of a semiconductor component which is attached to another material which functions, at least in part, as a heat sink or heat-conductive material. Such devices include, but are not limited to, those which contain one or more p-n junctions. Examples of such devices include laser diodes (as used herein, this term includes laser diode arrays), light-emitting diodes (as used herein, this term includes superluminescent diodes and superluminescent diode arrays), thyristors, triacs and microwave electron transfer devices. In order to minimize any thermally induced mechanical stress on the brittle semiconductor component of the device and to maximize the removal of heat from the semiconductor component of the device, it is required that the heat conductive material have the following properties: (1) A coefficient of thermal expansion which approximates that of the semiconductor component as closely as possible; and (2) The highest possible thermal conductivity.
An optimized heat conductive material is particularly important for use in the fabrication of semiconductor lasers. Such devices contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, can be constructed from a variety of semiconductor materials which include: (a) compounds such as GaAs and InP; (b) ternary alloys such as AlGaAs, GaAsP and InGaAs; and (c) quaternary alloys such as InGaAsP, InGaAlP and AlGaAsP. The efficiency of such lasers in converting electrical power to output optical radiation is relatively high and, for example, can be in excess of 50 percent. However, electrical power which is not converted to light by such a device is lost as heat.
Removal of by-product heat from a laser diode during operation is important since the lifetime of such a device is a strong function of operating temperature, with an increased operating temperature resulting in a reduced lifetime for the device. For example, the lifetime of a conventional gallium arsenide based laser diode will decrease by about an order of magnitude for a 40.degree. C. rise in temperature. Accordingly, simple heat transfer considerations suggest that a laser diode should be in contact with a heat sink material of the highest possible thermal conductivity in order to remove by-product heat as quickly as possible. However, if there is a mismatch between the coefficient of thermal expansion of the laser diode and that of the heat sink, the brittle semiconductor diode will experience mechanical stress during operation as a consequence of by-product heat production, and the magnitude of this stress will be a function of the size of the mismatch. Such stress can cause device failure within a few hours if it is severe and uncompensated.
The prior art has utilized two major approaches to the selection of a heat-conductive material for combination with a semiconductor. One approach has been to select a metal alloy or a composite structure which has a relatively high thermal conductivity and also has a coefficient of thermal expansion which is substantially identical with that of the semiconductor. For example, the coefficient of thermal expansion for GaAs is about 6.6.times.10.sup.-6 /.degree. C. and is well matched with that of a tungsten-copper alloy containing 85% by weight of tungsten (6.5.times.10.sup.-6 /.degree. C.) and of a molybdenum-copper alloy containing 85% by weight of molybdenum (6.6.times.10.sup.-6 /.degree. C.). U.S. Pat. No. 3,969,754 (Kuniya et al.; Jul. 13, 1976) discloses the use of a composite heat-conductive material for use in combination with a semiconductor where the composite is composed of a plurality of aligned fibers embedded in a metal matrix where the fibers have a coefficient of thermal expansion which is substantially equal to or lower than that of the semiconductor and the metal matrix has a thermal conductivity which is greater than that of the fibers. Similarly, U.S. Pat. No. 4,470,063 (Arakawa et al., Sept. 4, 1984) discloses the use of a composite of carbon fibers in a copper matrix as a conductor of heat and electricity for combination with a semiconductor. Unfortunately, this approach has not been entirely satisfactory because at least one member of the alloy or composite has had a relatively low thermal conductivity, which results in a lower than desirable overall thermal conductivity. With respect to the above-cited examples, the tungsten and molybdenum of the alloys and the fibers of the composites have relatively low thermal conductivities.
A second approach for the selection of a heat-conductive material for combination with a semiconductor has been to: (a) select a material which has an extremely high thermal conductivity, such as diamond, silver or copper, without regard for mismatch between the coefficients of thermal expansion, and (b) bond the heat-conductive and semiconductor materials together with a soft metal such as indium or a suitable soft metal alloy. In general, the larger the thermal coefficient of expansion mismatch between the heat-conductive material and the semiconductor, the softer the bond between the two must be. With respect to laser diodes, this approach is quite satisfactory for low power devices, for example about 100 mW or less, which produce relatively little heat during operation. Unfortunately, this approach is not satisfactory for higher power laser diodes.
The best heat sink materials from a thermal conductivity point of view include gold, silver, copper, aluminum and diamond. Unfortunately, these materials have a coefficient of thermal expansion which is very different from that of typical semiconductor materials. This will be illustrated by reference to the values which are set forth below in Table I.
TABLE I ______________________________________ Thermal Coefficient of Conductivity, Thermal Expansion, Material Watt/cm .multidot. .degree.C. 10.sup.-6 /.degree.C. ______________________________________ Semiconductors InP 0.80 4.75 GaP 1.1 5.91 GaAs 0.37 6.63 Ge 0.64 5.75 AlAs -- 5.20 InAs 0.29 5.16 Si 1.24 2.5 Metals Cu 4.0 16.6 Ag 4.3 19 Au 3.2 14.2 Al 2.4 25 Diamond Type Ia 9.9 0.8.sup.a Type IIa 23.2 0.8.sup.a Type IIb 13.6 0.8.sup.a ______________________________________ Note: .sup.a This value is reported for diamond at 20.degree. C. see KirkOthmer Encyclopedia of Chemical Technology, Vol. 4, 3rd edition, John Wiley & sons (1978), p. 669.
U.S. Pat. No. 2,382,666 (Rohrig et al.; Aug. 14, 1945) discloses the preparation of diamond composite tools by a process which involves: (a) coating small diamond particles with a metal film, for example by evaporation, glow discharge, cathodic sputtering or thermal decomposition of a metal carbonyl; and (b) incorporating the coated diamonds into a metal matrix. Similarly, U.S. Pat. No. 4,378,233 (Carver; Mar. 29, 1983) is directed to a grinding wheel which comprises a composite prepared by embedding diamond particles in a matrix which includes a mixture of aluminum, zinc, copper and tin and can also include up to 50% by volume of a dry film lubricant such as polytetrafluoroethylene, graphite, molybdenum disulfide and hexagonal boron nitride.